This invention relates to a glow discharge ionization source. More particularly, this invention relates to a glow discharge ionization source which operates at subatmospheric pressure and can directly accept a gaseous sample contained in ambient air. The invention relates specifically to apparatus and a method for detecting and analyzing a gaseous substance in an atmospheric sample.
Prior art systems have had the serious disadvantage of either requiring very large pumping systems and frequent maintenance or having low sensitivity and frequent clogging of the interface between the ion source and mass spectrometer. Since mass spectrometers operate under a high vacuum, it is difficult to obtain a sample of air which includes sufficient molecules of interest to detect without including a vast amount of unwanted atmospheric gases. Furthermore, it is also difficult to obtain indiscriminate ionization with prior art atmospheric ionization devices.
Attempts at analyzing dilute gas samples have produced unsatisfactory results. In atmospheric pressure ionization (API) sources, true thermal and chemical equilibria are obtained, so that only the most thermodynamically stable ions are observed. The less stable ions are, therefore, discriminated against severely. The relatively high operating pressure of the API source increases the probability of ion-ion recombination, which is a major loss mechanism, and also tends to increase the amount of clustering. Additionally, API sources are limited by either the amount of primary ionizing electrons, or the lifetime of the needle that produces the ionizing electrons. The former limitation leads to ionization saturation at relatively high sample concentrations, and discrimination in ionization when two components are present in widely varying concentrations. The kinetics of ion formation also contribute to ion discrimination. The limited lifetime of the needle that produces ionizing electrons precludes using this type of source for routine, continuous long-term applications.
In using an API source, the sample material passes through a region where ionizing electrons are generated. The ionizing electrons can be generated either by beta particle emission from a suitable material (e.g. .sup.63 Ni or .sup.3 H) or by establishing a corona discharge between two electrodes, one of which is a needle. After interacting with the ionizing electrons, the ionized molecules and their charged products are directed into a mass analyzer.
The corona discharge, which occurs at atmospheric pressure, cannot operate for long periods of time before maintenance is required. The discharge needle must be replaced every twenty-four to forty-eight hours due to burnout. This is a disadvantage for both remote monitoring systems and continuously operating systems.
Moreover, conventional API sources use small apertures which increase the chance of aperture clogging and require the use of a complex "gas curtain." In this regard, U.S. Pat. No. 4,137,750 to French et al. discloses an apparatus for analyzing trace components using a gas curtain. The gas curtain is established between a sample gas reaction chamber and a mass analyzer. A sample gas containing trace components is ionized in the reaction chamber which is maintained at atmospheric pressure. The ionized trace components are caused to migrate through the gas curtain by an electric field and separated from the sample gas. The gas curtain must be maintained at a pressure above atmospheric pressure so that no sample gas leaks into the mass analyzer.
Additionally, in the prior art the sensitivity of an API source using a beta emitter has not been adequate for trace atmospheric gas sampling. This is due to the radioactive nature of the beta emitter, which limits the amount that can safely be handled and thus, the number of ionizing electrons.
A different ionization source, chemical ionization (CI), operates at subatmospheric pressure but also requires frequent rebuilding of the electron source, as noted with the corona discharge API source. CI is primarily used for organic chemical analysis and is not used for direct atmospheric sampling.
Glow discharge ionization sources for mass spectrometers have heretofore been used only for inorganic chemical analysis. Typically, it is required that an electrode be made of a solid sample, such as a solid inorganic substance. Examples of conventional ion source electrode configurations are illustrated in W. W. Harrison et al., "Glow Discharge Mass Spectrometry," Analytical Chemistry, Volume 58, No. 2, pages 341A to 356A, February, 1986.
There is a need in the art for an ionization source for use with an atmospheric gas sampling instrument having high sensitivity and reduced tendency to discriminate against less stable ions. In addition, there is a need for a portable monitoring system that can operate in remote areas for extended periods of time before maintenance is required. There is also a need for an ionization source which can accept a gaseous sample directly from the environment and operate at subatmospheric pressure.